drm/i915: Wait for reset to complete before returning from debugfs/i915_wedged

Provide some serialisation between user operations by waiting for the
reset initiated by setting i915_wedged to complete.

The automatic wait here makes
        echo 1 > i915_wedged; cat i915_error_state
do the right thing, and not risk reporting "No error collected".

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170316171305.12972-4-chris@chris-wilson.co.uk